The present invention is directed to a temperature compensated bandgap voltage reference.
FIG. 1 shows how a reference voltage based upon Vbe of a bipolar transistor can be obtained. The current source I is provided in the emitter path of a bipolar transistor. A plurality of current sources can be provided each coupled to an FET of varying size to provide current sources of different magnitude, e.g., I, 10I, etc. as shown.
Vbe of a bipolar transistor decreases with increasing temperature in a well-known fashion. See FIG. 3. It is also known that a current mirror can be used to obtain a voltage representative of ΔVbe i.e., the difference between the Vbe of two bipolar transistors. FIG. 2 shows such a current mirror circuit. ΔVbe is equal to Vbe2 minus Vbe1 and ΔVbe is equal to kt/q ln NI/I. ΔVbe depends upon the ratio of the currents of the current sources as well as the temperature. In particular, ΔVbe increases with temperature. See FIG. 3. By combining the two circuits, it is possible to compensate Vbe of a first transistor with ΔVbe obtained via two other transistors Q1 and Q2, to obtain a substantially constant reference voltage Vref as shown in FIG. 3. In particular, Vref is equal to a constant A times Vbe plus a constant B times ΔVbe.